Estimated Heating Rates Due to Cyclotron Damping of Ion-scale Waves Observed by Parker Solar Probe

TL;DR

This study uses plasma dispersion modeling to analyze ion-scale waves observed by Parker Solar Probe, demonstrating their role in proton heating and anisotropy through cyclotron damping.

Contribution

It provides the first detailed evaluation of ion cyclotron wave damping rates at kinetic scales using a linear dispersion solver, linking wave observations to proton heating.

Findings

Ion-scale waves observed during 20.37% of PSP encounters.

Wave frequencies are consistent with transient ion cyclotron waves.

Estimated wave dissipation aligns with observed proton heating and anisotropy.

Abstract

Circularly polarized waves consistent with parallel-propagating ion cyclotron waves (ICWs) and fast magnetosonic waves (FMWs) are often observed by Parker Solar Probe (PSP) at ion kinetic scales. Such waves damp energy via the cyclotron resonance, with such damping expected to play a significant role in the enhanced, anisotropic heating of the solar wind observed in the inner heliosphere. We employ a linear plasma dispersion solver, PLUME, to evaluate frequencies of ICWs and FMWs in the plasma rest frame and Doppler-shift them to the spacecraft frame, calculating their damping rates at frequencies where persistently high values of circular polarization are observed. We find such ion-scale waves are observed during $20.37\%$ of PSP Encounters 1 and 2 observations and their plasma frame frequencies are consistent with them being transient ICWs. We estimate significant ICW dissipation onto protons, consistent with previous empirical estimates for the total turbulent damping rates, indicating that ICW dissipation could account for the observed enhancements in the proton temperature and its anisotropy with respect to the mean magnetic field.